Page 124 - Earth's Climate Past and Future

100 PART II • Tectonic-Scale Climate Change

0 The most important climatic record in the ocean is
the oxygen isotope signal (see Appendix 1 for a full
summary). Most of the oxygen in nature occurs as the
very abundant O isotope or as the much less abundant
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10
18 O isotope. Scientists refer to changes in the relative
amounts of these two isotopes as variations in δ O,
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measured as changes in parts per thousand (‰).
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Typical modern δ O values are 0 to –2‰ for the
surface ocean, +3 to +4‰ for the deep ocean, and –30
Myr ago 30 to –55‰ for ice sheets (Figure 6-6). Changes occur
through time in the δ O values of water in the ocean
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and of ice in the glaciers. In this chapter the main focus
is on changes in δ O values in the ocean. Foraminifera
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40 living both in surface waters and on the seafloor use
HCO ions dissolved in seawater as the source of car-
–
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bon and oxygen for their CaCO shells. Because the
3
–
50 oxygen in HCO comes directly from seawater, it gives
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climate scientists information on past variations in the
two isotopes of oxygen in the ocean, as quantified by the
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60 δ O signal.
10 15 20 25 30 In the past, δ O variations have been produced by
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Estimated mean annual temperature (˚C)
two climatic factors: (1) changes in the temperature of
FIGURE 6-5 Cooling in western North America ocean water and (2) changes in the size of ice sheets on
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Temperature trends estimated from the outline shapes of fossil the continents. Changes in δ O values measured in the
leaves indicate an erratic but progressive cooling of northern foraminiferal shells decrease by 1‰ for each 4.2°C
middle latitudes during the last 55 Myr. (Adapted from J. A. increase in the temperature of ocean water at the loca-
Wolfe, “Tertiary Climatic Changes in Western North America,” tion where the foraminifera lived. The same relation-
Palaeogeography, Palaeoclimatology, Palaeoecology 108 [1994]: ship holds if seawater cools but in the reverse sense
195–205.) (δ O values become heavier).
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Changes in size of the ice sheets also alter δ O val-
ues in foraminiferal shells. Ice sheets are formed from
have irregular edges, jagged or serrated in outline. The water vapor evaporated from the oceans and later pre-
reason for this relationship is not known, but the corre- cipitated as snow (Appendix 1). Because the snow and
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lation with temperature in the modern vegetation is ice are enriched in the lighter O isotope, more of the
strong enough that climate scientists have used this heavier O isotope is left behind in the oceans. This
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relationship to estimate past temperatures from assem- enrichment process is called fractionation. As a result,
blages of fossil leaves preserved in sedimentary rocks. the δ O value of ocean water becomes more positive as
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One record derived from leaf-margin evidence in ice sheets grow.
western North America shows an ongoing cooling over
the last 50 Myr (Figure 6-5). Although interrupted by
small warm intervals, the trend toward ever-lower tem-
peratures persists over the long term.
Poles Middle latitudes Tropics
6-2 Evidence from Oxygen Isotope Measurements Ice
–30 –55
Evidence of climate change on the continents over the River –15 Surface 0 –2
ocean
last 50 Myr is incomplete. The first occurrences of ice
sheets on land and their subsequent fluctuations in Deep ocean +3 +4
size are difficult to define. In addition, lakes that accu-
mulate remains of past continental vegetation in their
muddy sediments rarely persist for millions of years. In FIGURE 6-6 Typical δ O values in the modern world
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contrast, parts of the deep ocean have accumulated a δ O values in the ocean vary from 0 to –2‰ in warm tropical
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continuous climatic record with quantitative informa- surface waters today to as much as +3 to +4‰ in cold deep-
tion about climate change across the entire 50-Myr ocean waters. In today’s ice sheets, typical δ O values reach
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interval. –30‰ in Greenland and –55‰ in Antarctica.